arXiv subjects

Shinji Machida

Publications and source records attributed to Shinji Machida.

6 recordsLinked to original sources

arXiv2025

Characterisation and mitigation of RF knockout during beam stacking

Beam stacking allows a Fixed Field alternating gradient Accelerator (FFA) to increase the extracted beam current whilst also allowing for a flexible time structure making FFAs a promising candidate for future spallation neutron sources and high beam intensity applications. For successful beam stacking, beam loss caused by RF knockout must be avoided. RF knockout can occur during beam stacking because of the finite dispersion function at the RF cavity location, which is unavoidable in a scaling FFA. In this work, the RF knockout resonance is characterised and through a series of experiments at the ISIS Neutron and Muon Source, we show that it is possible to suppress the loss from RF knockout.

physics.acc-ph↗

arXiv2022

Review of coupled betatron motion parametrizations and applications to strongly coupled lattices

The coupling of transverse motion is a natural occurrence in particle accelerators, either in the form of a residual coupling arising from imperfections or originating by design from strong systematic coupling fields. While the first can be treated perturbatively, the latter requires a robust approach adapted to strongly coupled optics and a parametrization of the linear optics must be performed to explore beam dynamics in such peculiar lattices. This paper reviews the main concepts commonly put forth to describe coupled optics and clarifies the proposed parametrization formalisms. The links between the generalized Twiss parameters used by the different approaches are formally proven, and their physical interpretations are highlighted. The analytical methods have been implemented in a reference Python package and connected with a ray-tracing code to explore strongly coupled lattices featuring complex 3D fields. Multiple examples are discussed in detail to highlight the key physical interpretations of the parametrizations and characteristics of the lattices.

physics.acc-ph↗

arXiv2020

Identification and characterization of high order incoherent space charge driven structure resonances in the CERN Proton Synchrotron

Space charge is typically one of the performance limitations for the operation of high intensity and high brightness beams in circular accelerators. In the Proton Synchrotron (PS) at CERN, losses are observed for vertical tunes above $Q_y=6.25$, especially for beams with large space charge tune shift. The work presented here shows that this behaviour is associated to structure resonances excited by space charge due to the highly symmetric accelerator lattice of the PS, typical for first generation alternating gradient synchrotrons. Experimental studies demonstrate the dependency of the losses on the beam brightness and the harmonic of the resonance, and simulation studies reveal the incoherent nature of the resonance. Furthermore, the calculation of the Resonance Driving Terms (RDT) generated by the space charge potential shows that the operational working point of the PS is surrounded by multiple space charge driven incoherent resonances. Finally, measurements and simulations on both lattice driven and space charge driven resonances illustrate the different behaviour of the beam loss depending on the source of the resonance excitation and on the beam brightness.

physics.acc-ph↗

arXiv2018

A study of coherent and incoherent resonances in high intensity beams using a linear Paul trap

In this paper we present a quantitative measurement of the change in frequency (tune) with intensity of four transverse resonances in a high intensity Gaussian beam. Due to the non-linear space charge forces present in high intensity beams, particle motion cannot be analytically described. Instead we use the Simulator of Particle Orbit Dynamics (S-POD) and the Intense Beam Experiment (IBEX), two linear Paul traps, to experimentally replicate the system. In high intensity beams a coherent resonant response to both space charge and external field driven perturbations is possible, these coherent resonances are excited at a tune that differs by a factor $C_{m}$ from that of the incoherent resonance. By increasing the number of ions stored in the linear Paul trap and studying the location of four different resonances we extract provisional values describing the change in tune of the resonance with intensity. These values are then compared to the $C_{m}$ factors for coherent resonances. We find that the $C_{m}$ factors do not accurately predict the location of resonances in high intensity Gaussian beams. Further insight into the experiment is gained through simulation using Warp, a particle-in-cell code.

physics.acc-ph↗

arXiv2017

Scaling Fixed-Field Alternating-Gradient accelerators with reverse bend and spiral edge angle

A novel scaling type of Fixed-Field Alternating-Gradient (FFAG) accelerator is proposed that solves the major problems of conventional scaling FFAGs. This scaling FFAG accelerator combines reverse bending magnets of the radial sector type and a spiral edge angle of the spiral sector type to ensure sufficient vertical focusing without relying on extreme values of either parameter. This new concept makes it possible to design a scaling FFAG for high energy (above GeV range) applications such as a proton driver for a spallation neutron source and an accelerator driven subcritical reactor.

physics.acc-ph↗

arXiv2013

Fixed field alternating gradient

The concept of a fixed field alternating gradient (FFAG) accelerator was invented in the 1950s. Although many studies were carried out up to the late 1960s, there has been relatively little progress until recently, when it received widespread attention as a type of accelerator suitable for very fast acceleration and for generating high-power beams. In this paper, we describe the principles and design procedure of a FFAG accelerator.

physics.acc-ph↗